Unsaturated polymers have wide commercial application. For some applications, however, it is desirable to remove the double bonds in the unsaturated polymers because these double bonds tend to degrade by oxidative, thermal, and photochemical pathways. When this unsaturation is removed by hydrogenation, materials possessing greatly improved stability and weatherability are normally obtained. They then can be processed and used at higher temperatures, making them even more commercially attractive.
Many unsaturated polymers, such as polybutadiene, polyisoprene and polydiene-styrene block copolymers are made by anionic polymerization techniques. As a result, they possess narrow molecular weight distributions with the polymers being blocky in nature. For many applications, this is a critical attribute of their performance. Therefore, any hydrogenation method used to improve the stability and weatherability of such materials should cause as little chain degradation or alteration as possible. In the case of styrene/diene block copolymers, particularly, it is particularly important to be able to selectively hydrogenate the ethylenically unsaturated portion of the molecule without substantially reducing the aromatic portion of the molecule.
Experience has shown that it is difficult to hydrogenate these polymers without side reactions that cause molecular weight degradation or gelation, and also in the case of the styrene block copolymers, without substantial hydrogenation of the aromatic rings in the styrene portion of the polymer.
In addition to the foregoing problems, hydrogenation is normally accomplished by use of gaseous hydrogen in the presence of either heterogeneous or homogeneous catalysts. The transportation, storage and delivery, of course, of gaseous hydrogen does require extreme caution due to its flammability and explosivity when mixed with air or oxygen.
Accordingly, it is an object of the present invention to provide a process for hydrogenating unsaturated polymers in the absence of added gaseous hydrogen.
It is another object of the present invention to provide a process for hydrogenating unsaturated polymers without substantial molecular weight degradation, gelation or crosslinking of the unsaturated polymer.
It is another object of the present invention to provide a method for hydrogenating unsaturated diene styrene copolymers without substantial reduction of the aromatic content of the polymers.